FIG. 1 illustrates a nacelle 100 that is used to house a typical turbofan propulsion engine for a commercial airliner. The nacelle 100 provides aerodynamic surfaces for smooth flow of air into and around the engine, and helps create a bypass air duct for the fan air, and may include a thrust reverser to slow the aircraft. The nacelle 100 may be fabricated using various types of structures and materials. For example, some of the nacelle 100 structures may be formed from metal or composite sandwich panels having an inner skin and an outer skin, and a core material forming many small cells between the skins, with the cell walls bonded or otherwise joined to the skins and typically positioned normal thereto. In strategic locations, some of the sandwich panels may constitute acoustic sandwich panels which damp noise emitted by the engine to quiet the aircraft and reduce output, community noise. Acoustic sandwich panels are sandwich panels with perforations formed in one of the skins, such that the individual cells of the core form resonating chambers for damping sound waves, in a known manner.
The nacelle 100 typically is comprised of, for example, an inlet cowl 104, a fan cowl 110, a thrust reverser 116, etc. Air enters the nacelle 100 at the inlet 104 where it is received/processed by a fan 122. At least a first portion of the air output by the fan 122 may be subjected to combustion by the engine. A second portion of the air output by the fan 122 might not undergo combustion (e.g., may bypass the engine).
As nacelle technology continues to mature, trends have indicated that the fan 122 size/diameter is increasing. The lower rotational speeds of a larger fan 122 are associated with a reduction in operating noise frequencies. The increase in the size of fan blades 122a also generate greater tonal noise levels. Conventionally, to accommodate the changes to the frequency/noise levels associated with the use of a larger fan 122, the thickness of acoustic sandwich panels is increased. This limits the feasibility to package acoustic sandwich structures within the most ideal or required nacelle loft lines. Therefore, there is a need in the art for an improved sound attenuation structure for an aircraft propulsion system that attenuates lower frequency noise without being excessively thick.